Belly tightning for hours.

I am 31 weeks today, I started to experience belly tightning lasting a couple of seconds and I know those are Braxton Hicks. Sometimes my belly gets rock hard and kind of painful for hours. It doesnt matter is I sit or stand, it stays hard and I cannot feel any baby movement during that time. It's like a constabt bracton hicks for hours. I mentioned to my OB today that my belly would get hard for a long period of time, but she didn't seem concerned. I didn't mention that it was kind of painful, mostly because it's annoying as he'll and gets uncomfortable. It happened again tonight, my belly was super tight for about an hour and then it hot better long enough for baby to kick me twice SUPER hard before it started again. It makes me worry that the reason why my baby kicked super hard after was because she couldn't get enough oxygen for a while and that it could lead to brain damage or something. Have you felt something similar and everything was fine? I'll see my OB only in 2 weeks for my next appointment and I cannot call the office to talk to her, they don't really do that.